Wet etching is the core technique for patterning metal film using acid etchant and forming Gates, Source-Drains, and Indium Tin Oxides (ITO) electrodes in thin-film transistor (TFT hereinafter) manufacturing process. Wherein, Aluminum and Molybdenum are often configured as conducting material to form the Gates, the etchant can be different kinds of acid, but the metal film is mostly dissolved, oxidized, and reduced by strong acid mixture (e.g., mixed phosphoric acid, phosphoric acid, and acetic acid), so that the Gate film is patterned. The acid mixture serving as the etchant is generally composed of phosphoric acid (70%-72%), nitric acid (1.8%-2.0%), and acetic acid (9.5%-10.5%). Wherein, the nitric acid is configured to supply H3O+ and oxidize the metal (e.g., Aluminum and Molybdenum) for etching, the phosphoric acid provides phosphate radical, and can form complex with the oxidized metal to dissolve the metal oxides, the acetic acid can stick to the surface of the reactants, and thereby reduce the viscosity of the etchant, increase invasion of the etchant, and adjust the etching rate. In the etching process for forming the Gates and the Source-Drains, the concentration of the acid mixture is to reduce gradually due to the constant consumption of the nitric acid and the acetic acid, and then the etching quality may be adversely affected. In order to avoid bad etching effect and maintain the etching rate and the etching quality, accurately controlling the concentration of the nitric acid and the acetic acid is very important. Therefore, additional nitric acid and acetic acid should be continually added into the acid mixture etchant for maintaining the compositional ratio of the acid mixture etchant in the etching process. Accordingly, the manufacturing process needs an online monitoring device configured to control the concentration of each kind of acid of the acid mixture etchant and thereby ensure an appropriate etching rate and good etching quality.
As shown in FIG. 1, a conventional online monitoring device generally uses an online optical concentration measuring mechanism based on Lambert-Beer theory to monitor the concentration of each kind of acid of the acid mixture etchant in the etching process. The measurement principle is as follow: a light source 210 emits light with multiple wavelengths; the light passes through a monochromator 220 to generate light with specific wavelengths; the light with the specific wavelengths passes through a reference cell 240 and a sample cell 230 containing the acid mixture, which are arranged in parallel; a detector 250 measures the difference between absorbance of the sample cell 230 and absorbance of the reference cell 240; and finally the concentration of each kind of acid of the acid mixture in the sample cell 230 is calculated using Lambert-Beer theory.
However, in an actual production, an optical concentration measuring mechanism is a sealed container. Compared with external pipes, the online optical concentration measuring mechanism is always in the state of negative pressure. The acid mixture transported into the sample cell 230 through acid transporting pipe (not labeled in figures) will inevitably slightly volatilize at the joint of the acid transporting pipe and the sample cell 230, especially low boiling acetic acid will volatilize seriously. Lingering acid mist will be formed in the sealed negative pressure system of the online optical concentration measuring mechanism over time, and the surface of the sample cell 230 or the surface of the reference cell 240 may be covered by the acid mist. Thus, light adsorption may be adversely affected, and measured value of the acid concentration may be adversely affected too. The measured value of the acid concentration may be lower or higher than the real value, which may result in that the acid supplement system complements excessive or insufficient acid, and the concentration of the acid in the etchant is too large or too small. Thus, a large quantity of etching defects may occur, which may need much rework and/or repair, and cause great losses in the production. In practical use, to overcome the acid mist phenomenon, it is necessary to dismantle the online optical concentration measuring mechanism or wipe the measuring cell periodically. These operations may destroy precision of the measuring mechanism, and the reassembled online optical concentration measuring mechanism needs to be recalibrated before being configured again, which brings a lot of inconvenience to the actual production.